1. Field of the Invention
This invention relates to the field of fiber mats including multicomponent fiber mats and processes of forming such mats.
2. Description of the Related Art
Fibers and nanofibers are finding new applications in the pharmaceutical, filter, catalysts, clothing, and medical industries. Techniques such as electrospinning have been used to form fibers and nanofibers. For example, electrospinning techniques have been used to form fibers as small as a few nanometers in a principal direction. The phenomenon of electrospinning involves the formation of a droplet of polymer at an end of a needle, the electric charging of that droplet in an applied electric field, and an extraction of the polymer material from the droplet into the environment about the tip such as to draw a fiber of the polymer material from the tip.
Glass fibers have been manufactured in a sub-micron range for some time. Small micron diameter fibers have been manufactured and used commercially for air filtration applications for more than twenty years. Polymeric melt blown fibers have recently been produced with diameters less than a micron. Several value-added nonwoven applications, including filtration, barrier fabrics, wipes, personal care, medical and pharmaceutical applications may benefit from the interesting technical properties of nanofibers and nanofiber webs. Electrospun nanofibers have a dimension less than 1 μm in one direction and preferably a dimension less than 100 nm in this direction. Nanofiber webs have typically been applied onto various substrates selected to provide appropriate mechanical properties and to provide complementary functionality to the nanofiber web. In the case of nanofiber filter media, substrates have been selected for pleating, filter fabrication, durability in use, and filter cleaning considerations, as described in U.S. Pat. No. 6,673,136, the entire contents of which are incorporated herein by reference.
Conventional techniques for electrospinning produce mats of fibers or nanofibers having a uniform chemical composition throughout the mat. Even if the electrospin medium (i.e., the liquid or dissolved polymer) is a mix of various polymers, the fibers produced would have a uniform composition at any given location in the resultant fiber mat, i.e., the composition at any point being determined by the polymer constituency at the time of electrospinning. In addition, the conventional electrospinnning techniques produce fibers of a uniform fiber thickness at any point in the resultant fiber mat, as factors preset on the electrospinning device such as for example the electric field strength and the drying rate determine the fiber thickness produced.
Recently, Smith et al in U.S. Pat. No. 6,753,454, the entire contents of which are incorporated herein by reference, describe a technique for electrospinning fibers simultaneously or sequentially from multiple polymer-containing reservoirs. In this technique, the reservoirs for electrospinning were connected via a switch to a common power supply generating the requisite electric field by which the fibers are electrospun. As such, the fibers electrospun from the separate reservoirs collect onto a common ground electrode. Smith et al describe one utility of an alloyed fiber mat in the field of medical dressings where one side of the fiber composite is predominantly a set of hydrophilic fibers and the other side is predominantly a set of hydrophobic fibers. Smith et al also describe a polymer membrane forming the medical dressing that is generally formulated from a plurality of fibers electrospun from a substantially homogeneous mixture of any of a variety of hydrophilic and at least weakly hydrophobic polymers, that can be optionally blended with any of a number of medically important wound treatments, including analgesics and other pharmaceutical or therapeutical additives. For example, Smith et al describe polymeric materials suitable for electrospinning into fibers that may include absorbable and/or biodegradable polymeric substances that react with selected organic or aqueous solvents, or that dry quickly. Smith et al also describe that essentially any organic or aqueous soluble polymer or any dispersions of such polymer with a soluble or insoluble additive suitable for topical therapeutic treatment of a wound may be employed.
A schematic representation of the apparatus of Smith et al is shown in FIG. 1. FIG. 1 depicts an electrospinning apparatus 10 for the production of a fiber mat. The term “fiber mat” is used to define a plurality of fibers formed by forming fiber after fiber on each other. Respective fibers in the fiber mat can intermingle or be separate from other fibers in the fiber mat. Conventionally, the electrospinning apparatus 10 produces fibers that weakly adhere to each other.
The electrospinning apparatus shown in FIG. 1 is capable of producing fiber mats from separate electrospinning devices. The electrospinning apparatus 10 has two electrospinning devices 10a and 10b that each produces a same electric field 12 that extracts a polymer melt or solution 14 extruded from a tip 16 of an extrusion element 18 to a collection electrode 20. An enclosure/syringe 22 stores the polymer solutions 14 in each of the electrospinning devices 10a and 10b. A voltage power source 24 is electrically connected with one electrode through a wire 26 to each of the electrospinning devices 10a and 10b, and the other electrode of the power source 24 is electrically connected to ground. A switch 25 connects either of the electrospinning devices 10a and 10b to the power supply 24. The electric field 12 created between the tip 16 and the collection electrode 20 causes the polymer solution 14 to overcome cohesive forces that hold the polymer solution together. A jet of the substance 14 is drawn from the tip 16 toward the collection electrode 20 by the electric field 12 (i.e., electric field extracted), and dries during flight from the extrusion element 18 to the collection electrode 20 in a fiber extraction region 27 to form polymeric fibers, which can be collected downstream on the collection electrode 20.
However, fibers produced from the apparatus in FIG. 1 can suffer from poor adherence among the fibers that constitute the fiber mat due to the electrospun substances having the same electric polarities which in turn results in the collected fibers being repelled from each other as the fibers coalesce together on the collection electrode 20.